jayant gupchup phoenix, ewsn 2010 phoenix: an epidemic approach to time reconstruction jayant...

Jayant Gupchup Phoenix, EWSN 2010

Phoenix: An Epidemic Approach to Time Reconstruction

Jayant Gupchup†, Douglas Carlson†, Răzvan Musăloiu-E.†,* , Alex Szalay±, Andreas Terzis†

Department of Computer Science, Johns Hopkins University†

Department of Physics and Astronomy, Johns Hopkins University±

Google *

Jayant Gupchup Phoenix, EWSN 2010

where: Environmental Monitoring

Jayant Gupchup Phoenix, EWSN 2010

Design Goals and Targets

Target Lifetime : 1 yearo Duty-cycle (~ 5%)

Accuracy Requirementso Milliseconds (ms) - Seconds (s)o Online Synchronization not needed

Delay-tolerant networkso Basestation collects data opportunisticallyo NOT “sample-and-send”

All measurements require timestampso Not just events

Jayant Gupchup Phoenix, EWSN 2010

Naïve Time Reconstruction

Measurements are timestamped using motes local clock

Basestation collects data

Time reconstruction algorithm: Assigns measurements a global timestamp

Jayant Gupchup Phoenix, EWSN 2010

Reconstruction is NOT Synchronization

Asynchronous operationo Each mote has its own operation scheduleo No attempt to match schedules

Moteso Agnostic of network time / global timeo Do not process time informationo Do not have an onboard Real-Time Clock (RTC) (E.g. Telos, Mica2, MicaZ, IRIS)

Jayant Gupchup Phoenix, EWSN 2010

Phoenix Performance Accuracy

o Order of seconds, ~ 6 PPM (ignoring temperature effects)

Yield : Fraction of measurements assigned timestamps

o ≥ 99%

Overheads:o Duty-Cycle : 0.2% o Space : 4%

Yield performance maintained:o Presence of random, frequent mote rebootso Absence of global clock source for months

Jayant Gupchup Phoenix, EWSN 2010

Backgroundand

Related Work

Jayant Gupchup Phoenix, EWSN 2010

Reboots and Basestation

? ? ?

Jayant Gupchup Phoenix, EWSN 2010

Cub Hill – Year long deployment

Jayant Gupchup Phoenix, EWSN 2010

Cub Hill : Time Reconstruction

Nodes Stuck(Data Loss)

Watchdog Fix

Basestation Down

Reboot Problems

Jayant Gupchup Phoenix, EWSN 2010

Rate of Reboots

Jayant Gupchup Phoenix, EWSN 2010

Reconstruction Challenges Motes reboot at random

oDowntime is non-deterministic

Dependence on basestation

Temporary network partitions

Mote clocko Varies per moteo Skew changes over time

Jayant Gupchup Phoenix, EWSN 2010

Related Work Linear Regression for Time Rectification

o Fidelity and Yield in a Volcano Monitoring Sensor Network, Werner-Allen et al., OSDI 2006

Reboot Problemso Lessons from the Hogthrob Deployments, Chang et al., WiDeploy 2008o Trio: Enabling sustainable and scalable outdoor wireless sensor network

deployments, Dutta et al., SPOTS 2006

State preservation after rebootso Surviving sensor network software faults, Chen et al., SIGOPS 2009

Data-driven Temporal Integrityo Recovering temporal integrity with data driven time synchronization, Lukac et al.,

IPSN 2009o Sundial: Using sunlight to reconstruct global timestamps, Gupchup et al., EWSN

2009

Jayant Gupchup Phoenix, EWSN 2010

Phoenix

Jayant Gupchup Phoenix, EWSN 2010

Big Picture

1

2

3

Base Station

Jayant Gupchup Phoenix, EWSN 2010

Terminology Segment: State defined by a monotonically increasing local clock (LC)

o Comprises <moteid, reboot counter>

Anchor:o <local,neighbor> : Time-references between 2 segmentso <local,global> : Time-references between a segment and global time

Fit : Mapping between one time frame to anothero Defined over <local,neighbor> : Neighbor Fito Defined over <local,global> : Global fit

Fit Parameterso Alpha (α) : Skewo Beta (β) : Offset

Goodness of Fit : Metric that estimates the quality of the fito E.g. : Variance of the residuals

Jayant Gupchup Phoenix, EWSN 2010

2-Phase

Phase-I : Data Collection

(In-network)

Phase-II : Timestamp Assignment

(Database)

Jayant Gupchup Phoenix, EWSN 2010

Architecture Summary

Motes

Global Clock Source

Basestation

Jayant Gupchup Phoenix, EWSN 2010

Anchor Collection – I : Beaconing

Each Mote:

• Beacons time-state periodically• <moteid, RC #, LC>• Beacon interval~ 30s• Duty-cycle overhead: 0.075%

43 5 102400

97 7 3600

28 3 9600000

43

97

28

Jayant Gupchup Phoenix, EWSN 2010

Anchor Collection – II : Storage

43 5 102800

97 7 3800

Each Mote:

• Stays up (30s) after reboot • Listens for announcements• Wakes up periodically (~ 6 hrs)• Stays up (30s)• Listens for announcements

• Stores <local, neighbor> anchors

• Duty-Cycle : 0.14%

43

28

97

Jayant Gupchup Phoenix, EWSN 2010

Anchor Collection – III : Global References

97 7 4000 G-Mote:

• Connected to a global clock source• Beacon its time-state (30s)• Store Global References (6 hrs)• Global clock source (GPS, Basestation etc)

28 4 102435

28 4 102455, 1217351879

97

43

Jayant Gupchup Phoenix, EWSN 2010

43-5(B)

43-5(B)

97-7(A)

97-7(A)

28-4(G)

28-4(G)

97-7(A)

97-7(A)

43-5(B)

43-5(B)

28-4(G)

28-4(G)

Time Reconstruction (outside the network)

<Global Fit>

χ = 2

χ = 2.5

χ = 7

<Global Fit>

<Global Fit>

Segment Graph

Jayant Gupchup Phoenix, EWSN 2010

Evaluation:

Simulation & Experiments

Jayant Gupchup Phoenix, EWSN 2010

Evaluation Metrics Yield:

Fraction of samples assigned timestamps (%)

Average PPM Error: PPM Error per measurement:

Duty Cycle Overhead: Fraction of time radio was on (%)

Space Overhead: Fraction of space used to store anchors (%)

Jayant Gupchup Phoenix, EWSN 2010

Simulation: Missing Global Clock Source

Simulation Period : 1 Year

Jayant Gupchup Phoenix, EWSN 2010

Simulation: Wake Up Interval

Anchor collection rate should be significantly faster than the rate of reboots

Jayant Gupchup Phoenix, EWSN 2010

Simulation: Segments to anchor with

Jayant Gupchup Phoenix, EWSN 2010

Olin Deployment

- 19 Motes - 21 Day Deployment - 62 segments - One Global clock mote

Jayant Gupchup Phoenix, EWSN 2010

Deployment Accuracy

Jayant Gupchup Phoenix, EWSN 2010

Naïve Yield Vs Phoenix Yield

Phoenix Yield: 99.5%

Jayant Gupchup Phoenix, EWSN 2010

Conclusion Phoenix timestamps:

o > 99% of the collected measurementso With accuracy in order of seconds

Phoenix is Robust to:o Basestation failures for days-monthso Random mote reboots

Paying a price of:o 0.2% increase in duty cycleo 4% space overhead

Jayant Gupchup Phoenix, EWSN 2010

Questions ?

Jayant Gupchup Phoenix, EWSN 2010

Extra:

Jayant Gupchup Phoenix, EWSN 2010

Discussion / Future Work Choosing the right link metric

o Factor number of anchor pointso Temporal separation of anchorsoCombining the metrics along a “fit” path

Adaptive anchor collectiono If rate of reboots is unknown

Compare with online timestamping (FTSP)

Jayant Gupchup Phoenix, EWSN 2010

Simulation ParametersParameter Type Default Value

Clock Skew Uniform Distribution ~ U (40 70) [ppm]

Segment Model Non-Parametric (Cub Hill) median : 4 days

Topology Cub Hill (53 nodes)

Communication Delay(end-to-end)

Uniform Distribution ~ U (5 15) [ms]

Packet Reception Ratio Log-Normal Path Loss Pr(2.0) = -59.28 η = 2.04σ = 6.28

<beacon, Wakeup> Constant <30s, 6h>

<listen, synch> Constant <30s, 6h>

NUM_SEGMENTS Constant 4

Sampling Frequency(measurements)

Constant 10 mi

Jayant Gupchup Phoenix, EWSN 2010

Reboots: Long downtimes

Jayant Gupchup Phoenix, EWSN 2010

Clock Skews

Jayant Gupchup Phoenix, EWSN 2010

Temperature dependence

Source: http://focus.ti.com/lit/an/slaa322b/slaa322b.pdf